Semiconductor structures are inspected prior to, during, and after patterning procedures. Patterned metal films used in integrated circuit devices are often created using a damascene technique, in which a pattern is etched in an insulating dielectric layer, and subsequently filled using any of several standard deposition techniques, e.g., chemical vapor deposition (CVD), physical vapor deposition (PVD), or electro-copper plating (ECP). In the course of this process defects may be created inside or under the metal, such as voids, delamination, underfill or underetch of the dielectric, and other interface-related defects. Such buried defects are difficult to detect due to opacity of the surface layer.
U.S. Pat. No. 4,710,030 discloses use of a pump beam of short, non-destructive laser pulses (0.01–100 ps duration) to induce a thermo-elastic deformation, or stress waves, in a structure being tested, and to monitor the transient response of the structure using a low-power laser probe beam that is directed to the area of the deformation. By analyzing the intensity of the returning probe beam, information regarding defects and other characteristics of the structure can be inferred.
Besides reflections of short-pulse-induced stress waves, voids and interface defects are known to produce other physical effects in response to a pump beam, such as changes in acoustic dispersion properties, and reduced heat dissipation. These effects are discussed in an article entitled “Picosecond Ultrasonics” by Grahn et al., IEEE Journal of Quantum Electronics, Vol. 25 No. 12, pp. 2562–2568 (December 1989).
Moreover, U.S. Pat. No. 5,633,711 discloses monitoring a transient response to an excitation laser pulse that impinges on and locally heats a structure. In this disclosure, besides the intensity of the probe beam, phenomena such as acoustic oscillations and polarization disturbances are taken into account.
U.S. Pat. No. 6,320,666 discloses an intensity modulated pump laser beam, which is focused onto a sample so as to excite the sample periodically. Periodic heating by the pump beam creates a time varying deformation in the sample surface. A probe laser beam, obtained from a second laser, is focused onto the sample within the periodically heated area. A photodetector monitors the reflected power of the probe beam and generates an output signal responsively thereto. The output signal is filtered and processed to provide a measure of the modulated optical reflectivity of the sample.
U.S. Pat. No. 5,748,317 discloses the use of laser time-delayed pump and probe beams for determining the thermal properties of thin film. Measurements of reflectance and other optical characteristics are used to estimate the Kapitza resistance of a film. Inferences regarding the structure of the film or interfaces therein are made using reference data obtained from simulation or from another sample.
U.S. Pat. No. 6,253,621 discloses analysis of acoustic waves that are generated in a sample under test in response to a pulsed laser that is directed to a micro-spot on the sample and scanned. Acoustic waves are detected, and an acoustic index of refraction of a portion of the conductive structure is calculated as a function of the wave. The acoustic index of refraction is then spatially mapped over the sample.
U.S. Pat. No. 6,606,401 discloses detecting defects by comparing periodic structures, such as wafer dies. The signals for each wafer die are compared to at least the signals from the two nearest neighbor dies. Preferably the two wafer dies are located on either side of the die, in the same row as the die. See also U.S. Pat. No. 5,917,588 for a description of defect detection by analyzing a difference image among nearby reticle fields of a specimen wafer.
U.S. Pat. No. 5,131,755 discloses one configuration in which a substrate is compared to prestored expected characteristic features. Also disclosed is a second configuration in which first and second patterns in a region of the surface of at least one substrate are inspected by comparing one pattern against the other and noting where they agree with each other.
U.S. Pat. No. 6,297,879 discloses inspecting a photomask using a modified microscope, image comparison software and a CCD camera. The microscope is modified to view the photomask out of focus and at low magnifications. The image is compared with a reference image such as an image from another die or a database. Any discrepancies between images indicates a defect in the photomask.
U.S. Pat. No. 5,450,205 discloses measuring etching or deposition rate uniformity in situ using a CCD camera to view the wafer during plasma processing. The CCD camera records the temporal modulation of plasma emission or laser illumination reflected from the wafer; the modulation is caused by interferometry as thin films are etched or deposited. U.S. Pat. No. 5,450,205 also describes storing a time sequence of data signals corresponding to the signals generated by the CCD camera over a period of time. The storage is connected to data processing means for comparing the data signals for at least one sensing means of the array over a portion of the time period, and from comparisons determining the change of thickness of the film over the portion of the time period.
Each of the above-identified U.S. Patents is incorporated by reference herein in its entirety. Also incorporated by reference herein in its entirety is the above-identified article in IEEE Journal of Quantum Electronics.